1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a load-carrying cord for use in such power transmission belts, which cord has excellent tensile strength and resistance to both bending fatigue and fraying. The invention also relates to a method of making the fiber cord and a belt having the fiber cord incorporated therein.
2. Background Art
It is common to construct load-carrying cords in conventional V-belts by bundling fiber filaments and twisting a plurality of the bundled filaments to produce a cord of a desired thickness. The cord is then treated with any of a number of solutions that enhance its adherence to a rubber layer in which the cord is embedded.
It is also known to construct load-carrying cords from aramid fiber. Aramid fiber is desirable because of its strength, flexibility, and dimensional stability, even in high temperature environments. However, when the aramid fiber cords are treated with a solution to enhance their adherence to rubber, the solution penetrates the fibers and adversely affects the resistance of the cords to bending fatigue.
Another problem with cords made from aramid fiber is that they are prone to fraying. It is common in power transmission belts for the load-carrying cords to be exposed at one or both laterally oppositely facing side surfaces of the belt. This situation is common in toothed belts, V-ribbed belts, V-belts, etc. having exposed rubber sides surfaces that are not covered, as by fabric. These belts are commonly referred to in the industry as "raw edge belts".
Various solutions to the above problems have been attempted. In addition to addressing the above problems, belt designers have striven to maximize the tensile strength of the load-carrying cords.
It is known that the twist of the cord filaments affects the bending fatigue resistance for the cord. In Japanese Patent Laid Open Publication No. Sho 56-105,135, a specific range of twist coefficients is set forth to improve bending fatigue resistance.
An alternative proposed solution to the diminishing of bending fatigue resistance, by controlling twist, is disclosed in Japanese Patent Laid Open Publication No. Hei 2-42230. An improvement in bending fatigue resistance is disclosed by defining the cords using yarns with opposite primary and final twist directions. The final twist coefficient is described to be in the range of 3.5 to 5.7, with the catenary being no more than 0.8.
Generally, if the number of twists per unit length is increased, bending fatigue resistance for the fiber cord is improved. However, this is achieved at the expense of the tensile strength of the cord. As the number of twists per unit length is increased, the inclination of the fiber length to the length of the load-carrying cord is increased and, resultingly, the component of tensile force applied lengthwise of the fiber is decreased. Further, the alignment of the cord in a belt may be adversely affected when a large number of twists are used in defining the cords.
By modifying the cord twist to enhance bending fatigue resistance, the belt may also become more prone to fraying. Decreasing the number of twists per unit length increases the length of the fiber exposed at the side surfaces of the belt. During cutting of the belt, the cutter tends to fray the exposed cords. Cord fraying may be aggravated by pulleys cooperating with the belt in use, which may ultimately cause the cords to dislodge from the belt body. Various solutions have been proposed to this fraying problem.
One attempted solution to the fraying problem has been to treat the cords with a resorcin-formalin-rubber latex adhesive solution (RFL solution). Another proposed solution has been to pretreat the cords with epoxy or isocyanate compound and then use the above RFL adhering solution.
In the case of treatment with an RFL solution alone, bending resistance is improved, however, the problem of fraying remains. Pretreatment with the epoxy or isocyanate compound hardens the cords to reduce their propensity to fray. However, this pretreatment results in the deterioration of the bending fatigue resistance. Consequently, neither of the above proposed solutions satisfactorily addresses the problem of fraying while maintaining the desired bending characteristics of the cord.
Another improvement in load-carrying cords, focused on maximizing the tensile strength for load-carrying cords and reducing bending fatigue resistance and fraying, is disclosed in Japanese Patent Application No. Hei 3-133,419, owned by the assignee of the instant invention. Individual strands of untwisted aramid filaments, in a ribbon state and having a denier of 300-3100 d, are twisted with a primary twist. At least two such filaments are twisted and adhered, by epoxy resin, into bundles. The bundles are then twisted together with a final twist. The bundles can then be either a) adhered with rubber paste or b) treated additionally with an RFL solution prior to adherence with the rubber paste. The final twist coefficient is disclosed as being within the range of 1-4, with the primary twist coefficient being in the range of -1 to 1. The relationship between the final twist coefficient (X) and the primary twist coefficient (Y) is Y.gtoreq.-6.0X+1.3 Y is .ltoreq.-1.5X+5.0. This prior work was carried out by the assignee of this invention. The objective was to improve bending fatigue resistance, maximize tensile strength, and reduce fraying by a combination of controlling the cord twist and using an adhesive.
Another relevant disclosure is found in Japanese Patent Laid Open Publication No. Hei 2-42230. The final twist coefficient is characterized as being at least 5.0. If the primary twist coefficient is increased, the tensile strength of the cord is reduced, even if the ratio of the coefficients were to be 1.25 or less. There is still a substantial range of twist coefficients in which fraying is not adequately prevented by the prior art cords.